Study of Influence of Different Parameters of Head- Restraints on Energy Absorption Test

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2763
Issue 12, Volume 2 (December 2015) www.ijirae.com
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Study of Influence of Different Parameters of Head-
Restraints on Energy Absorption Test
Dhiraj DEVKATE*
, L.G. NAVALE
Dr.D.Y. Patil Institute of Engineering and Technology
Department of Mechanical Engineering, Pune University
Abstract—Head Restraints are provided to safeguard occupant head and neck from injuries during rear impact.
According to the study made by organizations like ECE (Economic commission of Europe) and NHTSA (National
Highway Traffic Safety Administration USA) on different accidents, many safety regulations are developed for
passenger cars coming on roads. ECER17 and FMVSS202a are the regulations made to test and ensure safety of seat
headrestraints. One of the most important tests according to these regulations is energy absorption test. In this test-
setup the Head Form with mass 6.8kg, diameter 165mm and velocity of 24.1km/hr hits the Head restraint. The passing
criteria of the test is such that the headform should not experience deceleration more than 80 times gravitation for the
period more than or equal to 3ms. Also during impact, there should not be excess lifting of the headrest. Different
parameters of the headrest assembly such as angle of inclination of the front face of headrest, radius of curvature of
headrest rod, Notch depth of notches on headrest rod and stiffness variation of the headrest plastic part have
influence on the test results. In this paper, influence of these parameters is studied on the energy absorption test
mainly on the lifting of the headrest.
Keywords—Headrestraint, ECER17, FMVSS202a
I. INTRODUCTION
During rear impact, the car moves in forward direction in a very short time span. During this impact the head moves
back instantly because of inertia. This sudden movement of head causes injury to cervical vertebra which is commonly
known as ‘Whiplash Injury’. Whiplash injury symptoms are often chronic problems that persist for years. Between 25
and 40 percent of whiplash injury victims never fully recover. Head restraints with a proper design can prevent whiplash
injury. The head restraints with good design should fulfill following requirements,
1. Support the head over sufficient area
2. Absorption of the head impact energy without losing structural integrity
3. No protruding edges during impact from head restraint
4. Reaction by head restraint should take care of deceleration of the head
5. No hard contact of head with any of the seat part
Regulations are made to ensure safety of occupants during car accidents. ECE and FMVSS are responsible for the
safety regulations in Europe and USA respectively. ECE stands for Economic Commission of Europe and FMVSS stands
for Federal Motor Vehicle Safety Standards. Regulations ECER17 and FMVSS202 are made to test Head restraint
assembly of a seat system of a vehicle. As per these regulations, static and dynamic tests are carried out on the seat head-
restraints.
In this study, the energy absorption simulation is done with the Ls-Dyna. The study is carried out on simplified model
of the seat structure. Head-restraint assembly parts like head-restraint rod, sleeves and locking mechanism are modeled
precisely to ensure accurate results. Strain rate dependent materials are used to predict exact behavior of the seat structure
as well as head-restraint assembly.
II. HARDWARE TEST SETUP
In this test, the spherical headform of diameter 165mm representing human head is made to hit seat head-restraint with
the velocity 24.1kmph and with energy 152.4J. The headform is made to hit the headrest above the plane which is 635
mm from H-point and normal to the torso of the seat and 70mm on either side of the median plane passing through head-
restraint.
Fig1:- Hardware test setup Fig 2:- Impact zone

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III. BEHAVIOUR OF THE HEAD-RESTRAINT ASSEMBLY DURING IMPACT
Fig 3:- Force diagram of the head-restraint assembly during impact
Basically the head-restraint rod is in contact with the plastic sleeve at only two locations. Numbers 1 and 2 in the
circle of fig3 shows the contact locations. So, Head-restraint rod moves inside the plastic sleeves and always remains in
contact at shown locations.
Whenever head-restraint is hit by head-form, the force applied by head-form can be considered to be resolved in to
global X and global Z direction. As shown in the fig3, force in the global Z direction is responsible for lifting of the
headrest and force in X direction is responsible to avoid excess lifting of the headrest. As shown in the figure3, the force
in the X direction is responsible to generate frictional force against the lifting of headrest as it increases normal reaction.
So, the study is oriented towards reducing lifting force and increasing normal reaction force on the head-restraint.
Hence to reduce lifting force, the influence of the stiffness variation of the plastic insert is studied by changing the
tooling direction of the plastic insert. Also, influence of other parameters is studied.
IV.FE MODEL SETUP
A. FE model setup to study influence of different parameters of Head-restraint assembly:-
To reduce the simulation time it was necessary to simplify seat model. Only metal tubes, metal sleeve reinforcement
bracket, plastic sleeves, headrest locking or adjustment mechanism and head-restraint rods of seat structure are used in
this study. The model setup is shown in figure 4.
Fig4:- Simplified FE model of seat structure
1. Modelling of Head-restraint rod:-
Head-restraint rods are meshed with the solid elements with very less average element length. All features of the
notches present on the head-restraint rod are captured. Steel material properties with strain rate effect are assigned to the
headrest rod.
2. Modelling of plastic sleeves:-
Plastic sleeves are the parts in which head-restraint rod fits. Plastic sleeve also facilitates the height adjustment of the
headrest as per user’s requirement. Hence, it is necessary to mesh the plastic sleeves precisely. Plastic sleeves are also
meshed with solid elements with very minimum element size.
Head-restraint rod
Front face of the
plastic insert
Plastic sleeves
Metal tubes
All DOF fixed (Connection of the seat structure to BiW)
1
2
Z
X

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Plastic sleeve also consists of locking mechanism which helps to hold head-restraint at user specified heights.
Locking mechanism is also represented in the model considering spring forces in it. Appropriate coefficient of friction is
defined between headrest rod and plastic sleeves.
3. Front face of the Headrest :-
Front face of the headrest is represented by a plane having angle of inclination similar to the angle defined in the
actual design of headrest. Shell elements are used to represent the front face of headrest with rigid material assigned.
Face is connected to top of the headrest rod by connection types used to connect rigid and elastic parts in Ls-Dyna.
4. Boundary Conditions:-
All DOF are fixed where the seat structure is connected to BiW. As shown in the fig 3.
B. FE model setup to study influence of direction of stiffness variation of head-restraint plastic
part :-
1. Overview:-
Head-restraint consists of plastic insert which is manufactured by injection moulding method. Depending upon
assembly process of plastic insert with head-restraint rod, the tooling direction of headrest is decided. To facilitate
easy removal of plastic part from mould, draft is given to ribs of plastic part. Hence more thickness is given to ribs at
the base and less thickness at the top. It results in to more stiffness of the plastic part at the base and lesser thickness
at the opening. In fig.4, the plastic insert is stiffer at the top and less stiff at the bottom.
Fig 4:- Original plastic insert having more stiffness at the top and less stiffness at the bottom
Here, the influence of direction of stiffness variation is studied by inverting tooling direction of the plastic insert and
by studying forces coming on the plastic insert by head-form. Fig.5 Shows plastic insert with inverted tooling direction.
Fig 5:- Plastic insert with inverted tooling direction.
2. Modelling of plastic insert.
The plastic insert is modelled by using 2-D mesh and capturing rib thickness variation along the height of the rib.
3. Boundary conditions
All DOF are constrained where Head restraint rod is clipped to the headrest plastic insert.
Tooling
direction
More stiff
region`
Less stiff
region
More stiffness at
bottom
Less stiffness at
Top

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4. Material
Plastic material with its behaviour in plastic region and strain rate is assigned to the plastic insert.
5. Loading:-
Impact load is applied to the plastic part face at the centre by a spherical head-form. The velocity of impact is adjusted to
get approximately same deformation of plastic insert as in the real cases.
Summary of study:-
TABLE I
LIST OF INFLUENCING PARAMETERS
Sr.
No.
Parameter for study Variable1 Variable2
1 Radius of curvature of headrest rod
Headrest rod with less
radius of curvature
Headrest rod with more radius of
curvature
2 Notch depth of headrest rod Original notch depth Notch depth increased by 0.2mm
3 Angle of inclination of front face of plastic insert As per existing design 10° +ve and 10°- ve.
4 Stiffness variation of plastic insert
More stiffness at top
than bottom
Less stiffness at the top than bottom
V. RESULTS
A. Influence of direction of variation of plastic insert.
1. Impact on the existing headrest plastic insert.
Fig 6 : Impact scenario on existing plastic insert and forces acting on it during impact.
It is observed from the force v/s time diagram that, force acting on the head-restraint in Z direction is positive. It
means there is a possibility of excess lifting of the headrest from seat.
2. Impact on plastic insert with inverted stiffness
Fig7 : Impact scenario on plastic insert with inverted stiffness and forces acting on it during impact.
It is observed from Fig. 7, The force applied by head-form on plastic insert in Z direction is negative for longer duration.
Hence it can be concluded that, after inverting stiffness of headrest plastic insert the possibility of excess lifting can be
reduced.

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B. Influence of notch depth of head-restraint rod on lifting
Fig 8: Influence of the increased notch depth of headrest rod on lifting
The notch depth can be increased by 0.2mm in the current design of headrest rod due to manufacturing limitations.
Hence, lifting of headrest rod with current design and headrest rod with increased notch depth are considered in the
study. It is found that from Fig8, there is no marginal influence of increased notch depth on lifting of headrest.
C. Influence of radius of curvature of headrest rod
Fig9 : Influence of radius of curvature of rod on lifting of headrest and deceleration of head-form
Currently two different headrest rods are being used in the seats having different radius of curvatures. The study is
carried out on same seat model but with different headrest rods. It is found that the Headrest rod having more radius
of curvature shows more lifting than headrest rod with less radius of curvature. But there is no much influence on the
deceleration of the head-form during impact.
D. Influence of the inclination of front face of headrest plastic insert.
Fig 10: Influence of angle of inclination of front face of plastic insert on lifting.
It is observed that, angle of inclination of front face of plastic insert has more influence on the lifting of the headrest.
It is required to keep less inclination of face of headrest plastic insert.
VI.CONCLUSIONS
1. If Stiffness of headrest plastic insert is more at the bottom and less at the top, then it helps to reduce the force in
upward direction which is responsible for lifting of head-restraint
2. The increased notch depth of head-restraint rod has no remarkable influence on lifting of the head-restraint.
3. Head-restraint rod with more radius of curvature shows more lifting of headrest. But there is no influence of
radius of curvature of head-restraint rod on deceleration of head-form.
4. Angle of inclination of headrest plastic insert has major influence on lifting of headrest.
5.

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ACKNOWLEDGMENT
Author would like to thank his Master program guide Dr. L.G. Navale, Master program coordinator Prof. Jamadar N.,
HOD of the department Dr. K.K. Dhande and all colleagues for their support during the study.
REFERENCES
[1] ECER17 Regulation
[2] http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r017r4e.pdf
[3] FMVSS 202 Regulation
[4] http://www.nhtsa.gov/cars/rules/rulings/HeadRest/PEA/Index.html
[5] Helen A. Kaleto, Herman W. Mangum, Peng Lee and Melanie Schick, Overview of Federal Motor Vehicle Safety
Standard (FMVSS) 202A–Head Restraints: Methodology and Equipment for the Dynamic Test, MGA Research
Corporation, 2007 World Congress, April 16-19, 2007
[6] Bhavin V. Mehta, Prasad Petkar and Robert Williams II, Importance of Seat and Head Restraint Positions in
Reducing Head-Neck Injuries, SAE Advances in Aviation Safety Conference, Seattle, Washington, September 10-
14, 2001
[7] John Gane , Measurement of Vehicle Head Restraint Geometry, International Congress and Exposition , Detroit
Michigan, March 1-4, 1999
[8] Eric R. Naber and Michael E. Carley, Expanded Polypropylene Foam as an Effective Solution to Passing FMVSS
202a, 2006 SAE world Congress, Detroit, April 3-6, 2006.
[9] Mats Y. Svensson1, Per Lövsund1, Yngve Håland2, Stefan Larsson3, The Influence of Seat-Back and Head-
Restraint Properties on the Head-Neck Motion During Rear-Impact, Dept. of Injury Prevention, Chalmers
University of Technology, S-412 96 Göteborg, Sweden
[10] Michael Kleinberger, Emily Sun, James Saunders and Zaifei Zhou3, Effects of head restraint position On neck
injury in rear impact, Traffic Safety and Auto Engineering Stream of the Whiplash-Associated Disorders World
Congress, 7-l 1 February 1999

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